The BRCA1 C-terminal domain: structure and function

Pathogenicity and Transcriptomic Analyses of Two "Candidatus Liberibacter asiaticus" Strains Harboring Different Types of Phages

"Candidatus Liberibacter asiaticus" is one of the putative causal agents of citrus Huanglongbing (HLB), a highly destructive disease threatening the global citrus industry. Several types of phages had been identified in "Ca. Liberibacter asiaticus" strains and found to affect the biology of "Ca. Liberibacter asiaticus." However, little is known about the influence of phages in "Ca. Liberibacter asiaticus" pathogenicity. In this study, two "Ca. Liberibacter asiaticus" strains, PYN and PGD, harboring different types of phages were collected and used for pathogenicity analysis in periwinkle (Catharanthus roseus). Strain PYN carries a type 1 phage (P-YN-1), and PGD harbors a type 2 phage (P-GD-2). Compared to strain PYN, strain PGD exhibited a faster reproduction rate and higher virulence in periwinkle: leaf symptoms appeared earlier, and there was a stronger inhibition in the growth of new flush. Estimation of phage copy numbers by type-specific PCR indicated that there are multiple copies of phage P-YN-1 in strain PYN, while strain PGD carries only a single copy of phage P-GD-2. Genome-wide gene expression profiling revealed the lytic activity of P-YN-1 phage, as evidenced by the unique expression of genes involved in lytic cycle, which may limit the propagation of strain PYN and lead to a delayed infection in periwinkle. However, the activation of genes involved in lysogenic conversion of phage P-GD-1 indicated it could reside within the "Ca. Liberibacter asiaticus" genome as a prophage form in strain PGD. Comparative transcriptome analysis showed that the significant differences in expression of virulence factor genes, including genes associated with pathogenic effectors, transcriptional factors, the Znu transport system, and the heme biosynthesis pathway, could be another major determinant of virulence variation between two "Ca. Liberibacter asiaticus" strains. This study expanded our knowledge of "Ca. Liberibacter asiaticus" pathogenicity and provided new insights into the differences in pathogenicity between "Ca. Liberibacter asiaticus" strains. IMPORTANCE Citrus Huanglongbing (HLB), also called citrus greening disease, is a highly destructive disease threatening citrus production worldwide. "Candidatus Liberibacter asiaticus" is one of the most common putative causal agents of HLB. Phages of "Ca. Liberibacter asiaticus" have recently been identified and found to affect "Ca. Liberibacter asiaticus" biology. Here, we found that "Ca. Liberibacter asiaticus" strains harboring different types of phages (type 1 or type 2) showed different levels of pathogenicity and multiplication patterns in the periwinkle plant (Catharanthus roseus). Transcriptome analysis revealed the possible lytic activity of type 1 phage in a "Ca. Liberibacter asiaticus" strain, which could limit the propagation of "Ca. Liberibacter asiaticus" and lead to the delayed infection in periwinkle. The heterogeneity in the transcriptome profiles, particularly the significant differences in expression of virulence factors genes, could be another major determinant of difference in virulence observed between the two "Ca. Liberibacter asiaticus" strains. These findings improved our understanding of "Ca. Liberibacter asiaticus"-phage interaction and provided insight into "Ca. Liberibacter asiaticus" pathogenicity.

BRCA1/2 variants of unknown significance in hereditary breast and ovarian cancer (HBOC) syndrome: looking for the hidden meaning

Hereditary breast and ovarian cancer syndrome is caused by germline mutations in BRCA1/2 genes. These genes are very large and their mutations are heterogeneous and scattered throughout the coding sequence. In addition to the above-mentioned mutations, variants of uncertain/unknown significance (VUSs) have been identified in BRCA genes, which make more difficult the clinical management of the patient and risk assessment. In the last decades, several laboratories have developed different databases that contain more than 2000 variants for the two genes and integrated strategies which include multifactorial prediction models based on direct and indirect genetic evidence, to classify the VUS and attribute them a clinical significance associated with a deleterious, high-low or neutral risk. This review provides a comprehensive overview of literature studies concerning the VUSs, in order to assess their impact on the population and provide new insight for the appropriate patient management in clinical practice.

Multifaceted Microcephaly-Related Gene MCPH1

MCPH1, or BRIT1, is often mutated in human primary microcephaly type 1, a neurodevelopmental disorder characterized by a smaller brain size at birth, due to its dysfunction in regulating the proliferation and self-renewal of neuroprogenitor cells. In the last 20 years or so, genetic and cellular studies have identified MCPH1 as a multifaceted protein in various cellular functions, including DNA damage signaling and repair, the regulation of chromosome condensation, cell-cycle progression, centrosome activity and the metabolism. Yet, genetic and animal model studies have revealed an unpredicted essential function of MPCH1 in gonad development and tumorigenesis, although the underlying mechanism remains elusive. These studies have begun to shed light on the role of MPCH1 in controlling various pathobiological processes of the disorder. Here, we summarize the biological functions of MCPH1, and lessons learnt from cellular and mouse models of MCPH1.

GC-MS Analysis, Molecular Docking and Pharmacokinetic Properties of Phytocompounds from Solanum torvum Unripe Fruits and Its Effect on Breast Cancer Target Protein

This study was designed to identify phytocompounds from the aqueous extract of Solanum torvum unripe fruits using GC–MS analysis against breast cancer. For this, the identified phytocompounds were subjected to perform molecular docking studies to find the effects on breast cancer target protein. Pharmacokinetic properties were also tested for the identified phytocompounds to evaluate the ADMET properties. Molecular docking studies were done using docking software PyRx, and pharmacokinetic properties of phytocompounds were evaluated using SwissADME. From the results, ten best compounds were identified from GC–MS analysis against breast cancer target protein. Of which, three compounds showed very good binding affinity with breast cancer target protein. They are ergost-25-ene-3,6-dione,5,12-dihydroxy-,(5.alpha.,12.beta.) (− 7.3 kcal/mol), aspidospermidin-17-ol,1-acetyl-16-methoxy (− 6.7 kcal/mol) and 2-(3,4-dichlorophenyl)-4-[[2-[1-methyl-2-pyrrolidinyl]ethyl amino]-6-[trichloromethyl]-s-triazine (− 6.7 kcal/mol). Further, docking study was performed for the synthetic drug doxorubicin to compare the efficiency of phytocompounds. The binding affinity of ergost-25-ene-3,6-dione,5,12-dihydroxy-,(5.alpha.,12.beta.) is higher than the synthetic drug doxorubicin (− 7.2 kcal/mol), and the binding affinity of other compounds is also very near to the drug. Hence, the present study concludes that the phytocompounds from the aqueous extract of Solanum torvum unripe fruits have the potential ability to treat breast cancer.

The fellowship of the RING: BRCA1, its partner BARD1 and their liaison in DNA repair and cancer

The breast cancer type 1 susceptibility protein (BRCA1) and its partner - the BRCA1-associated RING domain protein 1 (BARD1) - are key players in a plethora of fundamental biological functions including, among others, DNA repair, replication fork protection, cell cycle progression, telomere maintenance, chromatin remodeling, apoptosis and tumor suppression. However, mutations in their encoding genes transform them into dangerous threats, and substantially increase the risk of developing cancer and other malignancies during the lifetime of the affected individuals. Understanding how BRCA1 and BARD1 perform their biological activities therefore not only provides a powerful mean to prevent such fatal occurrences but can also pave the way to the development of new targeted therapeutics. Thus, through this review work we aim at presenting the major efforts focused on the functional characterization and structural insights of BRCA1 and BARD1, per se and in combination with all their principal mediators and regulators, and on the multifaceted roles these proteins play in the maintenance of human genome integrity.

Identification of Novel Inhibitors of Escherichia coli DNA Ligase (LigA)

Present in all organisms, DNA ligases catalyse the formation of a phosphodiester bond between a 3' hydroxyl and a 5' phosphate, a reaction that is essential for maintaining genome integrity during replication and repair. Eubacterial DNA ligases use NAD⁺ as a cofactor and possess low sequence and structural homology relative to eukaryotic DNA ligases which use ATP as a cofactor. These key differences enable specific targeting of bacterial DNA ligases as an antibacterial strategy. In this study, four small molecule accessible sites within functionally important regions of Escherichia coli ligase (EC-LigA) were identified using in silico methods. Molecular docking was then used to screen for small molecules predicted to bind to these sites. Eight candidate inhibitors were then screened for inhibitory activity in an in vitro ligase assay. Five of these (geneticin, chlorhexidine, glutathione (reduced), imidazolidinyl urea and 2-(aminomethyl)imidazole) showed dose-dependent inhibition of EC-LigA with half maximal inhibitory concentrations (IC₅₀) in the micromolar to millimolar range (11-2600 µM). Two (geneticin and chlorhexidine) were predicted to bind to a region of EC-LigA that has not been directly investigated previously, raising the possibility that there may be amino acids within this region that are important for EC-LigA activity or that the function of essential residues proximal to this region are impacted by inhibitor interactions with this region. We anticipate that the identified small molecule binding sites and inhibitors could be pursued as part of an antibacterial strategy targeting bacterial DNA ligases.

Single nucleotide polymorphisms in breast cancer susceptibility gene 1 are associated with susceptibility to lung cancer

BRCA1 is a tumor suppressor that has been found to be involved DNA synthesis during cell replication. In a recent study, the single nucleotide polymorphism (SNP), rs799917, in BRCA1 was found to be associated with the development and progression of various types of tumor. In the present study, the association between rs799917 and susceptibility to lung cancer was evaluated in a Han Chinese population in the Liaoning Province of China. The BRCA1 rs799917 genotypes (C/C, C/T and T/T) were analyzed using TaqMan quantitative PCR in 682 patients with lung cancer and 694 healthy controls, and the results were analyzed using a Student's t-test, a χ² test and logistic regression analysis. Individuals carrying the C/T or T/T genotype had a lower risk of lung cancer compared with those carrying the C/C genotype [odds ratio (OR), 0.741; P=0.021; and OR, 0.610; P=0.011, respectively). The C/T + T/T genotype group had an even lower risk (OR, 0.709; P=0.005) compared with that in the C/C genotype group. In the stratified analyses of non-smokers, individuals with the C/T or T/T genotype had a lower risk of developing lung cancer compared with that in those carrying the C/C genotype (OR, 0.681; P=0.013; and OR, 0.569; P=0.021, respectively). The stratified analyses of the BRCA1 rs799917 polymorphism based on pathological type, chemotherapy and radiotherapy, showed that in the squamous cell carcinoma, non-chemotherapy and non-radiotherapy subgroups, individuals with the T/T genotype had a lower risk of lung cancer compared with that in those carrying the C/C genotype (OR, 0.454; P=0.007; OR, 0.485; P=0.002; and OR, 0.599; P=0.020, respectively). In conclusion, the T allele of the rs799917 SNP in BRCA1 was associated with a lower risk of lung cancer in the ethnic Han Chinese population in Liaoning Province and may represent a protective factor against lung cancer.

BRCA1-A and BRISC: Multifunctional Molecular Machines for Ubiquitin Signaling

The K63-linkage specific deubiquitinase BRCC36 forms the core of two multi-subunit deubiquitination complexes: BRCA1-A and BRISC. BRCA1-A is recruited to DNA repair foci, edits ubiquitin signals on chromatin, and sequesters BRCA1 away from the site of damage, suppressing homologous recombination by limiting resection. BRISC forms a complex with metabolic enzyme SHMT2 and regulates the immune response, mitosis, and hematopoiesis. Almost two decades of research have revealed how BRCA1-A and BRISC use the same core of subunits to perform very distinct biological tasks.

Association of Genomic Domains in BRCA1 and BRCA2 with Prostate Cancer Risk and Aggressiveness

Pathogenic sequence variants (PSV) in BRCA1 or BRCA2 (BRCA1/2) are associated with increased risk and severity of prostate cancer. We evaluated whether PSVs in BRCA1/2 were associated with risk of overall prostate cancer or high grade (Gleason 8+) prostate cancer using an international sample of 65 BRCA1 and 171 BRCA2 male PSV carriers with prostate cancer, and 3,388 BRCA1 and 2,880 BRCA2 male PSV carriers without prostate cancer. PSVs in the 3' region of BRCA2 (c.7914+) were significantly associated with elevated risk of prostate cancer compared with reference bin c.1001-c.7913 [HR = 1.78; 95% confidence interval (CI), 1.25-2.52; P = 0.001], as well as elevated risk of Gleason 8+ prostate cancer (HR = 3.11; 95% CI, 1.63-5.95; P = 0.001). c.756-c.1000 was also associated with elevated prostate cancer risk (HR = 2.83; 95% CI, 1.71-4.68; P = 0.00004) and elevated risk of Gleason 8+ prostate cancer (HR = 4.95; 95% CI, 2.12-11.54; P = 0.0002). No genotype-phenotype associations were detected for PSVs in BRCA1. These results demonstrate that specific BRCA2 PSVs may be associated with elevated risk of developing aggressive prostate cancer. SIGNIFICANCE: Aggressive prostate cancer risk in BRCA2 mutation carriers may vary according to the specific BRCA2 mutation inherited by the at-risk individual.

Hereditary Breast and Ovarian Cancer Syndrome (BRCA) Gene: Concept, Pathways, Therapeutics, and Future

Hereditary breast and ovarian cancers are most commonly caused by mutations in BRCA1 and 2 genes. These are autosomal dominant mutations with high penetrance into subsequent generations. Affected individuals have deficiency in DNA repair mechanisms such as double strand DNA breaks (DSB) and non-homologous end joining (NHEJ). These tumors are peculiar due to early age of onset, typical histology such as triple negative breast cancers and high grade serous ovarian cancers and exquisite sensitivity to platinum analogues. These patients usually have better survival as compared to their wild type counterparts. Incidence of these mutations is rising due to better awareness about them amongst oncologists and patient population. Various genomic assays are available to detect germline and somatic BRCA mutations. Newer therapeutic frontiers like PARP inhibition have opened up due to better understanding of various mutations and their impact on subsequent pathways. Further studies are required to explore possibility of direct BRCA inhibition which may be useful in treatment of other solid organ cancers as well. This review focuses on understanding the pathophysiology of BRCA mutations, various pathways associated with the same, chemosensitivity patterns amongst affected cancer cells, targeted therapeutic opportunities and potential future developments in this field. We collected data from various published electronic records on google and have no conflicts of interest to be declared.

Mechanistic insight into the role of Poly(ADP-ribosyl)ation in DNA topology modulation and response to DNA damage

Genotoxic stress generates single- and double-strand DNA breaks either through direct damage by reactive oxygen species or as intermediates of DNA repair. Failure to detect and repair DNA strand breaks leads to deleterious consequences such as chromosomal aberrations, genomic instability and cell death. DNA strand breaks disrupt the superhelical state of cellular DNA, which further disturbs the chromatin architecture and gene activity regulation. Proteins from the poly(ADP-ribose) polymerase (PARP) family, such as PARP1 and PARP2, use NAD+ as a substrate to catalyse the synthesis of polymeric chains consisting of ADP-ribose units covalently attached to an acceptor molecule. PARP1 and PARP2 are regarded as DNA damage sensors that, upon activation by strand breaks, poly(ADP-ribosyl)ate themselves and nuclear acceptor proteins. Noteworthy, the regularly branched structure of poly(ADP-ribose) polymer suggests that the mechanism of its synthesis may involve circular movement of PARP1 around the DNA helix, with a branching point in PAR corresponding to one complete 360° turn. We propose that PARP1 stays bound to a DNA strand break end, but rotates around the helix displaced by the growing poly(ADP-ribose) chain, and that this rotation could introduce positive supercoils into damaged chromosomal DNA. This topology modulation would enable nucleosome displacement and chromatin decondensation around the lesion site, facilitating the access of DNA repair proteins or transcription factors. PARP1-mediated DNA supercoiling can be transmitted over long distances, resulting in changes in the high-order chromatin structures. The available structures of PARP1 are consistent with the strand break-induced PAR synthesis as a driving force for PARP1 rotation around the DNA axis.

A Novel Mutation-BRCA1 Associated Hereditary Haplotype of Intragenic Markers of BRCA1 Gene in a Family with History of Breast Cancer

Background: Breast cancer is the most common cancer diagnosed among women, Tumor suppressor genes such as BRCA1 involved in cell cycle control and repairing of DNA damage. BRCA1 is a risk factor gene that alteration in its protein cause in susceptibility to breast or ovarian cancer. Short tandem repeat (STR) polymorphism is linked to some disease. Objective: The aim of this study was screening a new mutation in patients with familial breast cancer. Materials and Methods: In this study, 200 women with breast cancer were participated. Among the patients, 40 women suffer from familial breast cancer. After DNA extraction from peripheral blood samples, Exons 16 to 23 of BRCA1 gene directly analyzed in SSCP gel electrophoresis followed by direct sequencing. Results: After direct sequencing, a new mutation was detected in intron 17 of BRCA1 gene. Three patients of one family have a germ line intronic mutation in the BRCA1 gene (IVS17-27delA). Also, this mutation in this family is linked to a haplotype of intragenic short tandem repeat (STR) in the BRCA1 gene. Conclusion: By Screening of gene mutations can be found association of mutation and incidence of disease. Also, studying the mutation in families and finding specific hereditary patterns in that family can be effective in prognosis of disease in other family members.

Antagonizing functions of BARD1 and its alternatively spliced variant BARD1δ in telomere stability

Previous reports have shown that expression of BARD1δ, a deletion-bearing isoform of BARD1, correlates with tumor aggressiveness and progression. We show that expression of BARD1δ induces cell cycle arrest in vitro and in vivo in non-malignant cells. We investigated the mechanism that leads to proliferation arrest and found that BARD1δ overexpression induced mitotic arrest with chromosome and telomere aberrations in cell cultures, in transgenic mice, and in cells from human breast and ovarian cancer patients with BARD1 mutations. BARD1δ binds more efficiently than BARD1 to telomere binding proteins and causes their depletion from telomeres, leading to telomere and chromosomal instability. While this induces cell cycle arrest, cancer cells lacking G2/M checkpoint controls might continue to proliferate despite the BARD1δ-induced chromosomal instability. These features of BARD1δ may make it a genome permutator and a driver of continuous uncontrolled proliferation of cancer cells.

Identification of a Variety of Mutations in Cancer Predisposition Genes in Patients With Suspected Lynch Syndrome

BACKGROUND & AIMS

Multigene panels are commercially available tools for hereditary cancer risk assessment that allow for next-generation sequencing of numerous genes in parallel. However, it is not clear if these panels offer advantages over traditional genetic testing. We investigated the number of cancer predisposition gene mutations identified by parallel sequencing in individuals with suspected Lynch syndrome.

METHODS

We performed germline analysis with a 25-gene, next-generation sequencing panel using DNA from 1260 individuals who underwent clinical genetic testing for Lynch syndrome from 2012 through 2013. All patients had a history of Lynch syndrome-associated cancer and/or polyps. We classified all identified germline alterations for pathogenicity and calculated the frequencies of pathogenic mutations and variants of uncertain clinical significance (VUS). We also analyzed data on patients' personal and family history of cancer, including fulfillment of clinical guidelines for genetic testing.

RESULTS

Of the 1260 patients, 1112 met National Comprehensive Cancer Network (NCCN) criteria for Lynch syndrome testing (88%; 95% confidence interval [CI], 86%-90%). Multigene panel testing identified 114 probands with Lynch syndrome mutations (9.0%; 95% CI, 7.6%-10.8%) and 71 with mutations in other cancer predisposition genes (5.6%; 95% CI, 4.4%-7.1%). Fifteen individuals had mutations in BRCA1 or BRCA2; 93% of these met the NCCN criteria for Lynch syndrome testing and 33% met NCCN criteria for BRCA1 and BRCA2 analysis (P = .0017). An additional 9 individuals carried mutations in other genes linked to high lifetime risks of cancer (5 had mutations in APC, 3 had bi-allelic mutations in MUTYH, and 1 had a mutation in STK11); all of these patients met NCCN criteria for Lynch syndrome testing. A total of 479 individuals had 1 or more VUS (38%; 95% CI, 35%-41%).

CONCLUSIONS

In individuals with suspected Lynch syndrome, multigene panel testing identified high-penetrance mutations in cancer predisposition genes, many of which were unexpected based on patients' histories. Parallel sequencing also detected a high number of potentially uninformative germline findings, including VUS.

Detection of single amino acid mutation in human breast cancer by disordered plasmonic self-similar chain

Control of the architecture and electromagnetic behavior of nanostructures offers the possibility of designing and fabricating sensors that, owing to their intrinsic behavior, provide solutions to new problems in various fields. We show detection of peptides in multicomponent mixtures derived from human samples for early diagnosis of breast cancer. The architecture of sensors is based on a matrix array where pixels constitute a plasmonic device showing a strong electric field enhancement localized in an area of a few square nanometers. The method allows detection of single point mutations in peptides composing the BRCA1 protein. The sensitivity demonstrated falls in the picomolar (10(-12) M) range. The success of this approach is a result of accurate design and fabrication control. The residual roughness introduced by fabrication was taken into account in optical modeling and was a further contributing factor in plasmon localization, increasing the sensitivity and selectivity of the sensors. This methodology developed for breast cancer detection can be considered a general strategy that is applicable to various pathologies and other chemical analytical cases where complex mixtures have to be resolved in their constitutive components.

Association between cytosolic expression of BRCA1 and metastatic risk in breast cancer

Background:

Although BRCA1 has been extensively studied for its role as a tumour-suppressor protein, the role of BRCA1 subcellular localisation in oncogenesis and tumour progression has remained unclear. This study explores the impact of BRCA1 mislocalisation on clinical outcomes in breast cancer.

Methods:

Tissue microarrays assembled from a cohort of patients with all stages of breast cancer were analysed for BRCA1 localisation and correlated with patient survival. Tissue microarrays of patients who had breast cancer that had metastasised to the lung were assembled from an independent cohort of patients. These were analysed for BRCA1 subcellular expression. In vitro studies using cultured human breast cancer cells were conducted to examine the effect of cytosolic BRCA1 on cell migration and efficiency of invasion.

Results:

An inverse association was found between cytosolic BRCA1 expression and metastasis-free survival in patients aged >40 years. Further analysis of BRCA1 subcellular expression in a cohort of breast cancer patients with metastatic disease revealed that the cytosolic BRCA1 content of breast tumours that had metastasised to the lung was 36.0% (95% CI=(31.7%, 40.3%), which was markedly higher than what is reported in the literature (8.2–14.8%). Intriguingly, these lung metastases and their corresponding primary breast tumours demonstrated similarly high cytosolic BRCA1 distributions in both paired and unpaired analyses. Finally, in vitro studies using human breast cancer cells demonstrated that genetically induced BRCA1 cytosolic sequestration (achieved using the cytosol-sequestering BRCA1 5382insC mutation) increased cell invasion efficiency.

Conclusions:

Results from this study suggest a model where BRCA1 cytosolic mislocalisation promotes breast cancer metastasis, making it a potential biomarker of metastatic disease.

Association of type and location of BRCA1 and BRCA2 mutations with risk of breast and ovarian cancer

IMPORTANCE

Limited information about the relationship between specific mutations in BRCA1 or BRCA2 (BRCA1/2) and cancer risk exists.

OBJECTIVE

To identify mutation-specific cancer risks for carriers of BRCA1/2.

DESIGN, SETTING, AND PARTICIPANTS

Observational study of women who were ascertained between 1937 and 2011 (median, 1999) and found to carry disease-associated BRCA1 or BRCA2 mutations. The international sample comprised 19,581 carriers of BRCA1 mutations and 11,900 carriers of BRCA2 mutations from 55 centers in 33 countries on 6 continents. We estimated hazard ratios for breast and ovarian cancer based on mutation type, function, and nucleotide position. We also estimated RHR, the ratio of breast vs ovarian cancer hazard ratios. A value of RHR greater than 1 indicated elevated breast cancer risk; a value of RHR less than 1 indicated elevated ovarian cancer risk.

EXPOSURES

Mutations of BRCA1 or BRCA2.

MAIN OUTCOMES AND MEASURES

Breast and ovarian cancer risks.

RESULTS

Among BRCA1 mutation carriers, 9052 women (46%) were diagnosed with breast cancer, 2317 (12%) with ovarian cancer, 1041 (5%) with breast and ovarian cancer, and 7171 (37%) without cancer. Among BRCA2 mutation carriers, 6180 women (52%) were diagnosed with breast cancer, 682 (6%) with ovarian cancer, 272 (2%) with breast and ovarian cancer, and 4766 (40%) without cancer. In BRCA1, we identified 3 breast cancer cluster regions (BCCRs) located at c.179 to c.505 (BCCR1; RHR = 1.46; 95% CI, 1.22-1.74; P = 2 × 10(-6)), c.4328 to c.4945 (BCCR2; RHR = 1.34; 95% CI, 1.01-1.78; P = .04), and c. 5261 to c.5563 (BCCR2', RHR = 1.38; 95% CI, 1.22-1.55; P = 6 × 10(-9)). We also identified an ovarian cancer cluster region (OCCR) from c.1380 to c.4062 (approximately exon 11) with RHR = 0.62 (95% CI, 0.56-0.70; P = 9 × 10(-17)). In BRCA2, we observed multiple BCCRs spanning c.1 to c.596 (BCCR1; RHR = 1.71; 95% CI, 1.06-2.78; P = .03), c.772 to c.1806 (BCCR1'; RHR = 1.63; 95% CI, 1.10-2.40; P = .01), and c.7394 to c.8904 (BCCR2; RHR = 2.31; 95% CI, 1.69-3.16; P = .00002). We also identified 3 OCCRs: the first (OCCR1) spanned c.3249 to c.5681 that was adjacent to c.5946delT (6174delT; RHR = 0.51; 95% CI, 0.44-0.60; P = 6 × 10(-17)). The second OCCR spanned c.6645 to c.7471 (OCCR2; RHR = 0.57; 95% CI, 0.41-0.80; P = .001). Mutations conferring nonsense-mediated decay were associated with differential breast or ovarian cancer risks and an earlier age of breast cancer diagnosis for both BRCA1 and BRCA2 mutation carriers.

CONCLUSIONS AND RELEVANCE

Breast and ovarian cancer risks varied by type and location of BRCA1/2 mutations. With appropriate validation, these data may have implications for risk assessment and cancer prevention decision making for carriers of BRCA1 and BRCA2 mutations.

MCPH1: a window into brain development and evolution

The development of the mammalian cerebral cortex involves a series of mechanisms: from patterning, progenitor cell proliferation and differentiation, to neuronal migration. Many factors influence the development of the cerebral cortex to its normal size and neuronal composition. Of these, the mechanisms that influence the proliferation and differentiation of neural progenitor cells are of particular interest, as they may have the greatest consequence on brain size, not only during development but also in evolution. In this context, causative genes of human autosomal recessive primary microcephaly, such as ASPM and MCPH1, are attractive candidates, as many of them show positive selection during primate evolution. MCPH1 causes microcephaly in mice and humans and is involved in a diverse array of molecular functions beyond brain development, including DNA repair and chromosome condensation. Positive selection of MCPH1 in the primate lineage has led to much insight and discussion of its role in brain size evolution. In this review, we will present an overview of MCPH1 from these multiple angles, and whilst its specific role in brain size regulation during development and evolution remain elusive, the pieces of the puzzle will be discussed with the aim of putting together the full picture of this fascinating gene.

Nuclear pore complex evolution: a trypanosome Mlp analogue functions in chromosomal segregation but lacks transcriptional barrier activity

The nuclear face of the nuclear pore complex (NPC) interfaces with chromatin, transcription, and transport intermediates. A novel architecture for the nuclear face of the trypanosome NPC provides insights into NPC function and evolution.

The nuclear pore complex (NPC) has dual roles in nucleocytoplasmic transport and chromatin organization. In many eukaryotes the coiled-coil Mlp/Tpr proteins of the NPC nuclear basket have specific functions in interactions with chromatin and defining specialized regions of active transcription, whereas Mlp2 associates with the mitotic spindle/NPC in a cell cycle–dependent manner. We previously identified two putative Mlp-related proteins in African trypanosomes, TbNup110 and TbNup92, the latter of which associates with the spindle. We now provide evidence for independent ancestry for TbNup92/TbNup110 and Mlp/Tpr proteins. However, TbNup92 is required for correct chromosome segregation, with knockout cells exhibiting microaneuploidy and lowered fidelity of telomere segregation. Further, TbNup92 is intimately associated with the mitotic spindle and spindle anchor site but apparently has minimal roles in control of gene transcription, indicating that TbNup92 lacks major barrier activity. TbNup92 therefore acts as a functional analogue of Mlp/Tpr proteins, and, together with the lamina analogue NUP-1, represents a cohort of novel proteins operating at the nuclear periphery of trypanosomes, uncovering complex evolutionary trajectories for the NPC and nuclear lamina.

Emerging roles of MCPH1: expedition from primary microcephaly to cancer

Genetic mutations in microcephalin1 (MCPH1) cause primary autosomal recessive microcephaly which is characterized by a marked reduction in brain size. MCPH1 encodes a centrosomal protein with three BRCT (BRCA1 C-terminal) domains. Also, it is a key regulator of DNA repair pathway and cell cycle checkpoints. Interestingly, in the past few years, many research studies have explored the role of MCPH1, a neurodevelopmental gene in several cancers and its tumor suppressor functions have been elucidated. Given the diverse new emerging roles, it becomes critical to review and summarize the multiple roles of MCPH1 that is currently lacking in the literature. In this review after systematic analysis of literature, we summarise the multiple functional roles of MCPH1 in centrosomal, DNA repair and apoptotic pathways. Additionally, we discuss the considerable efforts taken to understand the implications of MCPH1 in diseases such as primary microcephaly and its other emerging association with cancer and otitis media. The promising view is that MCPH1 has distinct roles and its clinical associations in various diseases makes it an attractive therapeutic target.

Alternative RNA splicing and cancer

Alternative splicing of pre-messenger RNA (mRNA) is a fundamental mechanism by which a gene can give rise to multiple distinct mRNA transcripts, yielding protein isoforms with different, even opposing, functions. With the recognition that alternative splicing occurs in nearly all human genes, its relationship with cancer-associated pathways has emerged as a rapidly growing field. In this review, we summarize recent findings that have implicated the critical role of alternative splicing in cancer and discuss current understandings of the mechanisms underlying dysregulated alternative splicing in cancer cells.

BRCA1--conductor of the breast stem cell orchestra: the role of BRCA1 in mammary gland development and identification of cell of origin of BRCA1 mutant breast cancer

Breast cancer treatment has been increasingly successful over the last 20 years due in large part to targeted therapies directed against different subtypes. However, basal-like breast cancers still represent a considerable challenge to clinicians and scientists alike since the pathogenesis underlying the disease and the target cell for transformation of this subtype is still undetermined. The considerable similarities between basal-like and BRCA1 mutant breast cancers led to the hypothesis that these cancers arise from transformation of a basal cell within the normal breast epithelium through BRCA1 dysfunction. Recently, however, a number of studies have called this hypothesis into question. This review summarises the initial findings which implicated the basal cell as the cell of origin of BRCA1 related basal-like breast cancers, as well as the more recent data which identifies the luminal progenitor cells as the likely target of transformation. We compare a number of key studies in this area and identify the differences that could explain some of the contradictory findings. In addition, we highlight the role of BRCA1 in breast cell differentiation and lineage determination by reviewing recent findings in the field and our own observations suggesting a role for BRCA1 in stem cell regulation through activation of the p63 and Notch pathways. We hope that through an increased understanding of the BRCA1 role in breast differentiation and the identification of the cell(s) of origin we can improve treatment options for both BRCA1 mutant and basal-like breast cancer subgroups.

BRCA1 and BRCA2 germline mutations in 85 Iranian breast cancer patients

Breast cancer is the most common cancer in Iranian women (Mousavi et al in Asian Pac J Cancer Prev 9(2):275-278, 2008). Genetic predisposition accounts for 15% of all breast cancers and germline mutations in breast cancer susceptibility genes, BRCA1 and BRCA2 are responsible for a substantial proportion of high-risk breast and breast/ovarian cancer families (Collaborative Group on Hormonal Factors in Breast Cancer in Lancet 350:1047-1059, 1997; Lee et al in Int Nurs Rev 55:355-359, 2008; Hulka and Stark in Lancet 346:883-887, 1995; Kelsey in Epidemiol Rev 15:256-263, 1993; Tischer et al in J Biol Chem 266:11947-11954, 1991; Newman et al in: Proc Natl Acad Sci USA 85:3044-3048, 1988). Therefore, the aim of this study was to investigate mutations of BRCA1/2 in high risk Iranian families. We screened 85 patients who met our minimal criteria. The entire coding sequences and each intron/exon boundaries of BRCA1/2 genes were screened by direct sequencing. In the present study, we could detect the novel following mutations: p.Glu1735 p.Gly1140Ser, p.Ile26Val, p.Leu1418X, p.Glu23Gln, p.Leu3X, p.Asn1403His, p.Lys581X, p.Pro938Arg, p.Thr77Arg, p.Arg7Cys, p.Ser177Thr, IVS7+83(TT), IVS8-70(-CATT), IVS2+9(-GC), IVS1-20(-GA), IVS1-8(-AG), IVS2+24(AG), IVS5-8 (A-G), IVS2(35-39)TTcctatGAT in BRCA1 and p.Glu1391Gly, 1994_1995 (Ins A), IVS6-70-T>G in BRCA2. In agreement with findings in other populations, we found that family history is a good predictor of being a mutation carrier. Five pathogenic BRCA1 mutations and one pathogenic BRCA2 mutation were detected in 85 index cases.

Two Missense Mutations in the Primary Autosomal Recessive Microcephaly Gene MCPH1 Disrupt the Function of the Highly Conserved N-Terminal BRCT Domain of Microcephalin

Primary microcephaly MCPH1 is an extremely rare autosomal recessive disorder associated with congenital microcephaly, mental retardation and a distinctive cellular phenotype of misregulated chromosome condensation. The MCPH1 gene encodes an 835-amino acid protein, microcephalin, which contains 1 N-terminal and 2 C-terminal BRCT (BRCA1 C-terminus) domains. BRCT domains are predominantly found in proteins involved in cell cycle control and DNA repair. Here we describe 1 novel and 1 previously reported MCPH1 missense mutation, p.Trp75Arg and p.Ser72Leu, respectively, in the N-terminal BRCT domain of microcephalin associated with severe congenital microcephaly. Both residues are entirely conserved in the MCPH1 orthologs of all vertebrate species and Drosophila. Proliferating lymphocytes of the patients with p.Trp75Arg and p.Ser72Leu show the unique cellular MCPH1 phenotype of misregulated chromosome condensation, indicating that these missense alterations disrupt the function of the N-terminal BRCT domain of the protein. Interestingly, both residues are strictly conserved in BRCT domains of BRCA1. ClustalW alignments show that the residue p.Ser72 of microcephalin corresponds to p.Ser1715 of the N-terminal BRCT domain of BRCA1, while the microcephalin residue p.Trp75 is analogous to p.Trp1718 in the N-terminal BRCT and to p.Trp1837 in C-terminal BRCT domains of BRCA1. Missense alterations for all 3 corresponding BRCA1 residues were described and are predicted to be deleterious resulting in the destabilization of the BRCA1 protein. Our data on the 2 MCPH1 missense alterations provide further evidence for the functional significance of these residues in BRCT domains.

The FN3 and BRCT motifs in the exomer component Chs5p define a conserved module that is necessary and sufficient for its function

Chs5p is a component of the exomer, a coat complex required to transport the chitin synthase Chs3p from the trans-Golgi network to the plasma membrane. The Chs5p N-terminal region exhibits fibronectin type III (FN3) and BRCT domains. FN3 domains are present in proteins that mediate adhesion processes, whereas BRCT domains are involved in DNA repair. Several fungi--including Schizosaccharomyces pombe, which has no detectable amounts of chitin--have proteins similar to Chs5p. Here we show that the FN3 and BRCT motifs in Chs5p behave as a module that is necessary and sufficient for Chs5p localization and for cargo delivery. The N-terminal regions of S. cerevisiae Chs5p and S. pombe Cfr1p are interchangeable in terms of Golgi localization, but not in terms of exomer assembly, showing that the conserved function of this module is protein retention in this organelle and that the interaction between the exomer components is organism-specific.

Functional Evolution of BRCT Domains from Binding DNA to Protein

The BRCT domain (BRCA1 C-terminal domain) is an important signaling and protein targeting motif in the DNA damage response system. The BRCT domain, which mainly occurs as a singleton (single BRCT) or tandem pair (double BRCT), contains a phosphate-binding pocket that can bind the phosphate from either the DNA end or a phosphopeptide. In this work, we performed a database search, phylogeny reconstruction, and phosphate-binding pocket comparison to analyze the functional evolution of the BRCT domain. We identified new BRCT-containing proteins in bacteria and eukaryotes, and found that the number of BRCT-containing proteins per genome is correlated with genome complexity. Phylogeny analyses revealed that there are two groups of single BRCT domains (sGroup I and sGroup II) and double BRCT domains (dGroup I and dGroup II). These four BRCT groups differ in their phosphate-binding pockets. In eukaryotes, the evolution of the BRCT domain can be divided into three phases. In the first phase, the sGroup I BRCT domain with the phosphate-binding pocket that can bind the phosphate of nicked DNA invaded eukaryotic genome. In the second phase, the phosphate-binding pocket changed from a DNA-binding type to a protein-binding type in sGroup II. The tandem duplication of sGroup II BRCT domain gave birth to double BRCT domain, from which two structurally and functionally distinct groups were evolved. The third phase is after the divergence between animals and plants. Both sGroup I and sGroup II BRCT domains originating in this phase lost the phosphate-binding pocket and many evolved protein-binding sites. Many dGroup I members were evolved in this stage but few dGroup II members were observed. The results further suggested that the BRCT domain expansion and functional change in eukaryote may be driven by the evolution of the DNA damage response system.

The structural basis for partitioning of the XRCC1/DNA ligase III-α BRCT-mediated dimer complexes

The ultimate step common to almost all DNA repair pathways is the ligation of the nicked intermediate to form contiguous double-stranded DNA. In the mammalian nucleotide and base excision repair pathways, the ligation step is carried out by ligase III-α. For efficient ligation, ligase III-α is constitutively bound to the scaffolding protein XRCC1 through interactions between the C-terminal BRCT domains of each protein. Although structural data for the individual domains has been available, no structure of the complex has been determined and several alternative proposals for this interaction have been advanced. Interpretation of the models is complicated by the formation of homodimers that, depending on the model, may either contribute to, or compete with heterodimer formation. We report here the structures of both homodimer complexes as well as the heterodimer complex. Structural characterization of the heterodimer formed from a longer XRCC1 BRCT domain construct, including residues comprising the interdomain linker region, revealed an expanded heterodimer interface with the ligase III-α BRCT domain. This enhanced linker-mediated binding interface plays a significant role in the determination of heterodimer/homodimer selectivity. These data provide fundamental insights into the structural basis of BRCT-mediated dimerization, and resolve questions related to the organization of this important repair complex.

Establishment of a mouse model with misregulated chromosome condensation due to defective Mcph1 function

Mutations in the human gene MCPH1 cause primary microcephaly associated with a unique cellular phenotype with premature chromosome condensation (PCC) in early G2 phase and delayed decondensation post-mitosis (PCC syndrome). The gene encodes the BRCT-domain containing protein microcephalin/BRIT1. Apart from its role in the regulation of chromosome condensation, the protein is involved in the cellular response to DNA damage. We report here on the first mouse model of impaired Mcph1-function. The model was established based on an embryonic stem cell line from BayGenomics (RR0608) containing a gene trap in intron 12 of the Mcph1 gene deleting the C-terminal BRCT-domain of the protein. Although residual wild type allele can be detected by quantitative real-time PCR cell cultures generated from mouse tissues bearing the homozygous gene trap mutation display the cellular phenotype of misregulated chromosome condensation that is characteristic for the human disorder, confirming defective Mcph1 function due to the gene trap mutation. While surprisingly the DNA damage response (formation of repair foci, chromosomal breakage, and G2/M checkpoint function after irradiation) appears to be largely normal in cell cultures derived from Mcph1^gt/gt mice, the overall survival rates of the Mcph1^gt/gt animals are significantly reduced compared to wild type and heterozygous mice. However, we could not detect clear signs of premature malignant disease development due to the perturbed Mcph1 function. Moreover, the animals show no obvious physical phenotype and no reduced fertility. Body and brain size are within the range of wild type controls. Gene expression on RNA and protein level did not reveal any specific pattern of differentially regulated genes. To the best of our knowledge this represents the first mammalian transgenic model displaying a defect in mitotic chromosome condensation and is also the first mouse model for impaired Mcph1-function.

Function of TopBP1 in genome stability

Human DNA topoisomerase IIbeta-binding protein 1 (TopBP1) and its orthologues in other organisms are proteins consisting of multiple BRCT modules that have acquired several functions during evolution. These proteins execute their tasks by interacting with a great variety of proteins involved in nuclear processes. TopBP1 is an essential protein that has numerous roles in the maintenance of the genomic integrity. In particular, it is required for the activation of ATM and Rad3-related (ATR), a vital regulator of DNA replication and replication stress response. The orthologues from yeast to human are involved in DNA replication and DNA damage response, while only proteins from higher eukaryotes are also involved in complex regulation of transcription, which is related to cell proliferation, damage response and apoptosis. We review here the recent progress in research aimed at elucidating the multiple cellular functions of TopBP1, focusing on metazoan systems.

DNA polymerase mu interacts with a meiosis-specific RecA homolog Lim15 during meiosis in Coprinus cinereus

Meiosis is a fundamental process in eukaryotes. Homologous chromosomes are paired and recombined during meiotic prophase I, which results in variation among the gametes. However, the mechanism of recombination between the maternal and paternal chromosome is unknown. In this study, we report on the identification of interaction between Coprinus cinereus DNA polymerase mu (CcPol mu) and CcLim15/Dmc1, a meiosis-specific RecA-like protein, during meiosis. Interaction between these two proteins was confirmed using a GST-pull down assay. A two-hybrid assay revealed that the N-terminus of CcPol mu, which includes the BRCT domain, is responsible for binding the C-terminus of CcLim15. Furthermore, co-immunoprecipitation experiments indicate that these two proteins also interact in the crude extract of the meiotic cell. A significant proportion of CcPol mu and CcLim15 is shown to co-localize in nuclei from the leptotene/zygotene stage to the early pachytene stage during meiotic prophase I. Moreover, CcLim15 enhances polymerase activity of CcPol mu early in the reaction. These results suggest that CcPol mu might be recruited by CcLim15 and elongate the D-loop structure during homologous recombination in meiosis.

Many roads lead to primary autosomal recessive microcephaly

Autosomal recessive primary microcephaly (MCPH), historically referred to as Microcephalia vera, is a genetically and clinically heterogeneous disease. Patients with MCPH typically exhibit congenital microcephaly as well as mental retardation, but usually no further neurological findings or malformations. Their microcephaly with grossly preserved macroscopic organization of the brain is a consequence of a reduced brain volume, which is evident particularly within the cerebral cortex and thus results to a large part from a reduction of grey matter. Some patients with MCPH further provide evidence of neuronal heterotopias, polymicrogyria or cortical dysplasia suggesting an associated neuronal migration defect. Genetic causes of MCPH subtypes 1-7 include mutations in genes encoding microcephalin, cyclin-dependent kinase 5 regulatory associated protein 2 (CDK5RAP2), abnormal spindle-like, microcephaly associated protein (ASPM), centromeric protein J (CENPJ), and SCL/TAL1-interrupting locus (STIL) as well as linkage to the two loci 19q13.1-13.2 and 15q15-q21. Here, we provide a timely overview of current knowledge on mechanisms leading to microcephaly in humans with MCPH and abnormalities in cell division/cell survival in corresponding animal models. Understanding the pathomechanisms leading to MCPH is of high importance not only for our understanding of physiologic brain development (particularly of cortex formation), but also for that of trends in mammalian evolution with a massive increase in size of the cerebral cortex in primates, of microcephalies of other etiologies including environmentally induced microcephalies, and of cancer formation.

Multimodal assessment of protein functional deficiency supports pathogenicity of BRCA1 p.V1688del

Unequivocal discrimination between neutral variants and deleterious mutations is crucial for appropriate counseling of individuals with a BRCA1 or BRCA2 sequence change. An increasing number of variants of uncertain significance (VUS) are being identified, the unclassified biological effect of which poses clinical concerns. A multifactorial likelihood-based approach recently suggested disease causality for BRCA1 p.V1688del, a VUS recurrent in Italian breast/ovarian cancer families. Whether and how this single amino acid deletion in the BRCA1 COOH terminus (BRCT) domain affects the function of the mutant protein (DeltaValBRCA1) has not been elucidated. We undertook comprehensive functional characterization of DeltaValBRCA1, comprising comparative structural modeling, analysis of protein stability and associations, and analysis of DNA repair function. Our model predicted BRCT domain destabilization and folding disruption caused by BRCA1 p.V1688del. Consistently, the recombinant DeltaValBRCA1 was less stable than wild-type BRCA1 and, unlike the latter, failed to associate with BRIP1, CtIP, and Rap80 and to relocalize to sites of DNA damage. Yeast two-hybrid analysis revealed a compromised interaction with FHL2 and KPNA2, which is likely responsible for improper subcellular localization of DeltaValBRCA1. In addition, we found four new breast/ovarian cancer families of Italian ancestry who carried this sequence alteration. These results provide the first evidence of the effect of BRCA1 p.V1688del on protein stability and function, supporting the view that it is a deleterious mutation. Multimodal analyses like ours could advance understanding of tumor suppression by BRCA1 and ultimately contribute to developing efficient strategies for screening and characterization of VUS.

Hsp70 translocates to the nuclei and nucleoli, binds to XRCC1 and PARP-1, and protects HeLa cells from single-strand DNA breaks

For many years, there has been uncertainty concerning the reason for Hsp70 translocation to the nucleus and nucleolus. Herein, we propose that Hsp70 translocates to the nucleus and nucleoli in order to participate in pathways related to the protection of the nucleoplasmic DNA or ribosomal DNA from single-strand breaks. The absence of Hsp70 in HeLa cells, via Hsp70 gene silencing (knockdown), indicated the essential role of Hsp70 in DNA integrity. Therefore, HeLa Hsp70 depleted cells were very sensitive in heat treatment and their DNA breaks were multiple compared to that of control HeLa cells. The molecular mechanism with which Hsp70 performs its role at the level of nucleus and nucleolus during stress was examined. Hsp70 co-localizes with PARP1 in the nucleus/nucleoli as was observed in confocal studies and binds to the BCRT domain of PARP1 as was revealed with protein-protein interaction assays. It was also found that Hsp70 binds simultaneously to XRCC1 and PARP-1, indicating that Hsp70 function takes place at the level of DNA repair and possibly at the base excision repair system. Making a hypothetical model, we have suggested that Hsp70 is the molecule that binds and interrelates with PARP1 creating the repair proteins simultaneously, such as XRCC1, at the single-strand DNA breaks. Our data partially clarify a previously unrecognized cellular response to heat stress. Finally, we can speculate that Hsp70 plays a role in the quality and integrity of DNA.

Y-family DNA polymerases in mammalian cells

Eukaryotic genomes are replicated with high fidelity to assure the faithful transmission of genetic information from one generation to the next. The accuracy of replication relies heavily on the ability of replicative DNA polymerases to efficiently select correct nucleotides for the polymerization reaction and, using their intrinsic exonuclease activities, to excise mistakenly incorporated nucleotides. Cells also possess a variety of specialized DNA polymerases that, by a process called translesion DNA synthesis (TLS), help overcome replication blocks when unrepaired DNA lesions stall the replication machinery. This review considers the properties of the Y-family (a subset of specialized DNA polymerases) and their roles in modulating spontaneous and genotoxic-induced mutations in mammals. We also review recent insights into the molecular mechanisms that regulate PCNA monoubiquitination and DNA polymerase switching during TLS and discuss the potential of using Y-family DNA polymerases as novel targets for cancer prevention and therapy.

Cell cycle regulation of the BRCA1/acetyl-CoA-carboxylase complex

Germ-line alterations in BRCA1 are associated with an increased susceptibility to breast and ovarian cancer. The BRCA1 protein has been implicated in multiple cellular functions. We have recently demonstrated that BRCA1 reduces acetyl-CoA-carboxylase alpha (ACCA) activity through its phospho-dependent binding to ACCA, and further established that the phosphorylation of the Ser1263 of ACCA is required for this interaction. Here, to gain more insight into the cellular conditions that trigger the BRCA1/ACCA interaction, we designed an anti-pSer1263 antibody and demonstrated that the Ser1263 of ACCA is phosphorylated in vivo, in a cell cycle-dependent manner. We further showed that the interaction between BRCA1 and ACCA is regulated during cell cycle progression. Taken together, our findings reveal a novel mechanism of regulation of ACCA distinct from the previously described phosphorylation of Ser79, and provide new insights into the control of lipogenesis through the cell cycle.

Evidence for a structural relationship between BRCT domains and the helicase domains of the replication initiators encoded by the Polyomaviridae and Papillomaviridae families of DNA tumor viruses

Studies of DNA tumor viruses have provided important insights into fundamental cellular processes and oncogenic transformation. They have revealed, for example, that upon expression of virally encoded proteins, cellular pathways involved in DNA repair and cell cycle control are disrupted. Herein, evidence is presented that BRCT-related regions are present in the helicase domains of the viral initiators encoded by the Polyomaviridae and Papillomaviridae viral families. Of interest, BRCT domains in cellular proteins recruit factors involved in diverse pathways, including DNA repair and the regulation of cell cycle progression. Therefore, the viral BRCT-related regions may compete with host BRCT domains for particular cellular ligands, a process that would help to explain the pleiotropic effects associated with infections with many DNA tumor viruses.

Structural evidence for direct interactions between the BRCT domains of human BRCA1 and a phospho-peptide from human ACC1

The tandem BRCA1 C-terminal (BRCT) domains are phospho-serine/threonine recognition modules essential for the function of BRCA1. Recent studies suggest that acetyl-CoA carboxylase 1 (ACC1), an enzyme with crucial roles in de novo fatty acid biosynthesis and lipogenesis and essential for cancer cell survival, may be a novel binding partner for BRCA1, through interactions with its BRCT domains. We report here the crystal structure at 3.2 A resolution of human BRCA1 BRCT domains in complex with a phospho-peptide from human ACC1 (p-ACC1 peptide, with the sequence 1258-DSPPQ-pS-PTFPEAGH-1271), which provides molecular evidence for direct interactions between BRCA1 and ACC1. The p-ACC1 peptide is bound in an extended conformation, located in a groove between the tandem BRCT domains. There are recognizable and significant structural differences to the binding modes of two other phospho-peptides to these domains, from BACH1 and CtIP, even though they share a conserved pSer-Pro-(Thr/Val)-Phe motif. Our studies establish a framework for understanding the regulation of lipid biosynthesis by BRCA1 through its inhibition of ACC1 activity, which could be a novel tumor suppressor function of BRCA1.

Identification of functional domains in TdIF1 and its inhibitory mechanism for TdT activity

TdT interacting factor 1 (TdIF1) was identified as a protein that binds to terminal deoxynucleotidyltransferase (TdT) to negatively regulate TdT activity. TdT is a template-independent DNA polymerase that catalyzes the incorporation of deoxynucleotides to the 3'-hydroxyl end of DNA templates to increase the junctional diversity of immunoglobulin or T-cell receptor (TcR) genes. Here, using bioinformatics analysis, we identified the TdT binding, DNA binding and dimerization regions, and nuclear localization signal (NLS) in TdIF1. TdIF1 bound to double-stranded DNA (dsDNA) through three DNA binding regions: residues 1-75, the AT-hook-like motif (ALM) and the predicted helix-turn-helix (HTH) motif. ALM in TdIF1 preferentially bound to AT-rich DNA regions. NLS was of the bipartite type and overlapped ALM. TdIF1 bound to the Pol beta-like region in TdT and blocked TdT access to DNA ends. In the presence of dsDNA, however, TdIF1 bound to dsDNA to release TdT from the TdIF1/TdT complex and to exhibit TdT activity, implying that active TdT released microenvironmentally concentrates around AT-rich DNA to synthesize DNA.

TheDrosophilahomolog ofMCPH1,a human microcephaly gene, is required for genomic stability in the early embryo

Mutation of human microcephalin (MCPH1) causes autosomal recessive primary microcephaly, a developmental disorder characterized by reduced brain size. We identified mcph1, the Drosophila homolog of MCPH1, in a genetic screen for regulators of S-M cycles in the early embryo. Embryos of null mcph1 female flies undergo mitotic arrest with barrel-shaped spindles lacking centrosomes. Mutation of Chk2 suppresses these defects, indicating that they occur secondary to a previously described Chk2-mediated response to mitotic entry with unreplicated or damaged DNA. mcph1 embryos exhibit genomic instability as evidenced by frequent chromatin bridging in anaphase. In contrast to studies of human MCPH1, the ATR/Chk1-mediated DNA checkpoint is intact in Drosophila mcph1 mutants. Components of this checkpoint, however, appear to cooperate with MCPH1 to regulate embryonic cell cycles in a manner independent of Cdk1 phosphorylation. We propose a model in which MCPH1 coordinates the S-M transition in fly embryos: in the absence of mcph1, premature chromosome condensation results in mitotic entry with unreplicated DNA, genomic instability, and Chk2-mediated mitotic arrest. Finally, brains of mcph1 adult male flies have defects in mushroom body structure, suggesting an evolutionarily conserved role for MCPH1 in brain development.

Microcephalin coordinates mitosis in the syncytialDrosophilaembryo

Microcephalin (MCPH1) is mutated in primary microcephaly, an autosomal recessive human disorder of reduced brain size. It encodes a protein with three BRCT domains that has established roles in DNA damage signalling and the cell cycle, regulating chromosome condensation. Significant adaptive evolutionary changes in primate MCPH1 sequence suggest that changes in this gene could have contributed to the evolution of the human brain. To understand the developmental role of microcephalin we have studied its function in Drosophila. We report here that Drosophila MCPH1 is cyclically localised during the cell cycle, co-localising with DNA during interphase, but not with mitotic chromosomes. mcph1 mutant flies have a maternal effect lethal phenotype, due to mitotic arrest occurring in early syncytial cell cycles. Mitotic entry is slowed from the very first mitosis in such embryos, with prolonged prophase and metaphase stages; and frequent premature separation as well as detachment of centrosomes. As a consequence, centrosome and nuclear cycles become uncoordinated, resulting in arrested embryonic development. Phenotypic similarities with abnormal spindle (asp) and centrosomin (cnn) mutants (whose human orthologues are also mutated in primary microcephaly), suggest that further studies in the Drosophila embryo may establish a common developmental and cellular pathway underlying the human primary microcephaly phenotype.

Thermal unfolding of human BRCA1 BRCT-domain variants

Missense mutations at the BRCT domain of human BRCA1 protein have been associated with an elevated risk for hereditary breast/ovarian cancer. They have been shown to affect the binding site and they have also been proposed to affect domain stability, severely hampering the protein's tumor suppressor function. In order to assess the impact of various such mutations upon the stability and the function of the BRCT domain, heat-induced denaturation has been employed to study the thermal unfolding of variants M1775R and R1699W, which have been linked with the disease, as well as of V1833M, which has been reported for patients with a family history. Calorimetric and circular dichroism results reveal that in pH 9.0, 5 mM borate buffer, 200 mM NaCl, analogously to wild type BRCT, all three variants undergo partial thermal unfolding to a denatured state, which retains most of the native's structural characteristics. With respect to wild-type BRCT, the mutation M1775R induces the most severe effects especially upon the thermostability, while R1699W also has a strong impact. On the other hand, the thermal unfolding of variant V1833M is only moderately affected relative to wild-type BRCT. Moreover, isothermal titration calorimetric measurements reveal that contrary to M1775R and R1699W variants, V1833M binds to BACH1 and CtIP phosphopeptides.

Rad4TopBP1 associates with Srr2, an Spc1 MAPK-regulated protein, in response to environmental stress

Rad4(TopBP1) is a scaffold in a protein complex containing both replication proteins and checkpoint proteins and plays essential roles in both replication and checkpoint responses. We have previously identified four novel fission yeast mutants of rad4+(TopBP1) to explore how Rad4(TopBP1), a single protein, can play multiple roles in genomic integrity maintenance. Among the four novel mutants, rad4-c17(TopBP1) is a thermosensitive mutant. Here, we characterized rad4-c17(TopBP1) and identified a rad4-c17(TopBP1) allele specific suppressor named srr2+ (suppressor of Rad4(TopBP1) R2 domain). srr2+ has previously been identified as an environmental stress-responsive gene (GenBank accession number AL049644.1, locus spcc191.01). srr2+ null cells are sensitive to hydroxyurea (HU) at elevated temperatures. Deletion of srr2+ in rad4-c17(TopBP1) exacerbates the HU sensitivity of the mutant. Overexpression of srr2+ suppresses the rad4-c17(TopBP1) mutant sensitivity to temperature and HU and restores the compromised ability of rad4-c17(TopBP1) to activating Cds1 kinase in response to HU treatment. Furthermore, stress-activated MAPK, Spc1 (also known as StyI or Phh1), induces the expression and phosphorylation of the Srr2 protein. Significantly, environmental stress induces co-precipitation of Srr2 protein with Rad4(TopBP1), and the co-precipitation is compromised in the rad4-c17(TopBP1) mutant. These results have led us to propose a model; Rad4(TopBP1) exists in a large protein complex to coordinate genomic perturbations with checkpoint responses to maintain genomic integrity. In addition, when cells experience environmental stress, Rad4(TopBP1) associates with Srr2, an Spc1 MAPK-responsive protein, to survive the stress, potentially by providing a link of the Spc1 MAPK response to checkpoint responses.

p53, BRCA1 and breast Cancer chemoresistance

The tumor suppressor genes p53 and BRCA1 are involved in hereditary as well as sporadic breast cancer development and therapeutic responses. While p53 mutations contribute to resistance to chemo- and radiotherapy, BRCA1 dysfunction leads to enhanced sensitivity to DNA damaging therapeutic agents. The biochemical pathways used by p53 and BRCA1 for signaling tumor suppression involve some cross-talk including repression of BRCA1 transcription by p53 and altered selectivity of p53-dependent gene activation by BRCA1. In this chapter we review clinical and preclinical data implicating p53 and BRCA1 in breast cancer chemosensitivity. We discuss the known signaling pathways downstream of p53 or BRCA1 that contribute to their modulation of therapeutic responses, and we discuss the implications of p53 or BRCA1 mutation in therapeutic design.

TopBP1 contains a transcriptional activation domain suppressed by two adjacent BRCT domains

TopBP1 has eight BRCT [BRCA1 (breast-cancer susceptibility gene 1) C-terminus] domains and is involved in initiating DNA replication, and DNA damage checkpoint signalling and repair. Several BRCT-domain-containing proteins involved in mediating DNA repair have transcriptional regulatory domains, and as demonstrated for BRCA1 these regulatory domains are important in mediating the functions of these proteins. These transcriptional regulatory processes involve modification of chromatin, and recent evidence has clearly demonstrated that the ability to modify chromatin plays an important role in regulating DNA damage signalling and repair. Here we report the identification of a TopBP1 transcriptional activation domain that is rich in hydrophobic residues, interspersed with acidic amino acids, characteristics that are typical of transcriptional activation domains identified previously. Two adjacent repressor domains encoded by BRCT2 and BRCT5 silence this activator and experiments suggest that these repressors actively recruit repressor complexes. Both the activator and BRCT2 repressor domains function in yeast. The present study identifies several chromatin modification domains encoded by TopBP1, and the implications of these findings are discussed in the context of the DNA damage response and the understanding of TopBP1 function.